This invention relates to indicator assemblies of the type which may be used as an indicator panel or in conjunction with an electric pushbutton switch wherein the indicator assembly may constitute a part of the pushbutton operator for the switch. More particularly, this invention relates to indicator assemblies of the aforementioned type which are sunlight readable (SR). Still more particularly, this invention relates to indicator assemblies of the aforementioned type which are sunlight readable (SR) and are back lit illuminated night vision goggle compatible (NVGC).
Indicator assemblies which are both sunlight readable (SR) and night vision goggle compatible (NVGC) have peculiar diverse operating requirements. Such indicator assemblies are commonly utilized in aircraft cockpits wherein strong ambient light on the order of 10,000 foot-candles can exist from bright sunlight. Thus, indicator assemblies intended for such applications must be capable of illumination at an intensity sufficient to clearly denote a contrast with the ambient light conditions. On the other hand, such indicator assemblies used in the same aircraft cockpit application may also be required to be NVG compatible.
In existing SR/NVGC devices, most of the visible and infrared wavelengths of the illumination source are filtered out, leaving only a small portion of the available light spectrum for illumination. As a result, in order to illuminate a legend at a desired intensity that will counteract or overcome very strong ambient light conditions, a strong (high intensity) light source must be utilized. Another concern is that panel area, often referred to as a "footprint", is limited and the space available for such high intensity lamp is quite small. Presently, however, no high intensity light source that has long life and a small footprint exists. High intensity light sources also generate a significant amount of heat, particularly within a confined area such as in an indicator panel assembly or in a pushbutton switch assembly. Heat generated by high intensity light sources can create a high touch temperature at the surface of the switch pushbutton. Such heat also significantly shortens the operating life of the illumination source.
Existing SR/NVGC indicators mainly rely on a variety of optical filters for contrast enhancement and color corrections. The use of filters of these types under intense ambient light and during an "OFF" state of the indicator introduces a phenomenon in which interfaces between the filtering layers create a so-called secondary image, or retroreflectivity, also called "flashback". Reduction of this phenomenon is mainly accomplished by giving particular attention to matching the index of refraction between the layers of filters by means of bonding (free of air bubbles) or laminating layers.
This invention overcomes the need for a high intensity light source in a sunlight readable indicator by covering the legend with an electronic light valve (shutter) which is closed when not indicating and open when indicting. The legend is translucent to permit light from a low intensity light source to pass therethrough, and is reflective of ambient light when the light valve is open, which coincides with energization of the light source. Thus the legend becomes illuminated by the stronger source of light, either the ambient light or the low intensity light source. Such light sources have a high mean time between failure (MTBF) rating and therefor increase the operating lifetime and reliability of the indicator assembly, as well as lower the power consumption and touch temperature at the surface.